Downgrade to 6.1.1 iOS version can only be achieved with the help of Redsn0w and the SHSH blobs you have saved for that iOS version. Read our FREE tutorial.
Jailbreak iPhone 4 with Redsn0w requires from you to have either a Windows Machine or a Mac OS machine. There is not a Linux Redsn0w version yet, but there is a way to run redsn0w on linux.
Redsn0w on Linux version is not available for you to download, but there are many ways that allow you to run windows software on Linux machines. Read More.
Skype application is a very powerful application which you can download and install to your iPhone 4, but users experienced sound issues during their calls.
Tweetbot is a mobile application that allows you to use Twitter to your iDevice, and there are special versions for your iPhone, iPad and iPod Touch.